Seems I've somehow fixed this issue, maybe others might find this useful.
Background, using an Asus DSL-N14U since March this year, and XB connected 100% first time - even had better streaming over wifi then my dlink modem. In short, worked perfectly.
Fast forward to April, go away for a month (so everything is shut down) come back and unable to connect to wifi. I tinker with the modem and change wireless to "Legacy mode" and it somehow connects to wifi. But not to Live. so it can browse my media library, but cant sign into live services.
Chat to MS live support online and we try
- remove persistent storage in bluray
- reset mac address
- power down and disconnect mains for a few minutes
- factory reset xbox (full data wipe & settings wipe)
- factory reset asus modem
- try connecting via LTE wifi hotspot on mobile
All failed. In fact, at this point - even when connecting through a wired connection it still could not go onto live. Now because we did a factory reset, the Xbox needs to download setup files which it simply refuses to do.
So i power down, disconnect power cable from XB and from wall socket. Power brick stops glowing orange after about 5minutes (i went out for some grub so was away for about an hour)
Now power up again and..
do one more factory reset, but via a USB stick
Download the reset files here
https://support.xbox.com/en-US/xbox-one/console/offline-system-update-diagnostic-tool
To boot of USB you need to disconnect any lan cables and
Press and hold the BIND button (located on the left side of the console) and the EJECT button (located on the front of the console), and then press the Xbox button on the console. You'll hear 2 beeps which indicate your files have been accepted by the console.
So it starts up, does a factory reset.
Power down again.
Then download the update file, there are 3 console versions, as luck would have it. Mine was OSUDT2 - 3.3gig.
I did try the other 2 but system kicked out an error code and failed.
Anyway you have to boot off USB about 4 times iirc. Each time it 'downloads' the files to the console from USB, restarts - then applies the files, restart again and boot of USB and download more files from usb, restart and install (i know - very very repetitive! )
Eventually brings up the XB introduction video, searches for wifi - finds and connects to my wifi in a snap. Sign into live, do your basic configurations and everything now works perfectly. Powered down and up again, and everything connected perfectly.
One last tearjerker.... i wanted a short wifi SSID and named it..... Hodor